Signal transmission system



July 28, 1931. R. c. MATHES SIGNAL TRANSMISSION SYSTEM Filed March 29, 1929- 2 Sheets-Sheet l /NVENTO/- E. C. MA1-Hes BY l ATTORNEY July 28, 1931.

R. c. MATHES SIGNAL TRANSMISSION SYSTEM File-d March 29 1929 2 Sheets-Sheet 2 #VVE/v70@ f?. CI MATHES A TTORNEY Patented July 28, 1931 A UNITED STATES PATENT OFFICE ROBERT o. MATIIEs, or WYOMING, NEW JERSEY, AssIGNOR To BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OE NEW YORK SIGNAL TRANSMISSION SYSTEM Application filed March 29, 1929. Serial No. 350,971.

This invention relates to signal transmission systemsv and particularly to circuits for controlling the energy levels in transmission systems.

5" VOne object of the invention lis to provide a signal transmission system with improved means forl and method of maintaining a constant maximum peak energy level and compressing the volume range of the siglo'nalsat the transmitting station.

Another object of the invention is to provide a signal transmission system, having signals of substantially the same volume range and different average levels supplied 15to the transmitting station, with means that shall maintain a constant maximum peak energy level and compress the volume range of the signals at the transmitting station.

` `A further object of the invention is to prov2o`^vide a signal transmission system of the above indicated character that shall maintain a constant maximum peak energy level and compress the volume range of the sigg 4nals at the transmitting station andv that shall expand the volume range of the signals at the receiving station.

In a signal `transmission system, signals which have very different average volume u levels are 1n many cases received atA the transmitting station.V The significant inomentary volume range in most casesvisfsubstantially the same but may vary. The difference in the average volume level of theY i received signals may be caused by toll lines4 in telephone systems of different electrical lengths and by Vspeakers who talk loudly and softly. The limitations in the transmission of signals caused `by apparatus or'.

line conditions may render it diiiicult to transmit to aV distant receiving station signals which have very different average volume l levels. The difference in average volume levels may be the result of any of vthe above mentioned causes. Y

lVhenl a telephone system is connected to a radio system suchl as in the trans-Atlantic.

radio system it becomes necessary to transmit over the radio system'at the highestpeak level possible because the noise or static level is particularly high and the IQCQived',

in other transmission systems having like problems to be solved. One solution of the problem is by means of the so-called volume control circuit and another solution of the problem is by the so-called volume compression circuit. A volume control circuit may be defined as a circuit which is adapted toV maintain at a particular point in a system a constant maximum peak level of power regardless of the level at which the speech f power may be received at that particular point in the system. A volume compression circuit may bevdefined as a circuit adapted to reduce the momentary volume range or to reduce the diiference between the maximum peak level and the minimuml significant peak level of signals.

The diiliculties produced by reason of the diii'erences in the average volume levels of signals may be reduced either by a volume control system or a volume compression system. The limitations however in some cases may be such that neithera volume control nor a volume compression system will com- Vpletely solve the problem. In accordance with the present invention it is proposed to move the average volume levels of the signals produced by a weak talker and a loud talker to the same level. When the signals have been moved to the same volume level, the volume range `is compressed according to the limitations of the system. If desired the volume ranges of the different signals may first be compressed and then the signals may be moved to have a constant maximum peak level. At the receiving end of the system it may in some cases be necessary to provide a volume expander to reverse the operation of the volume compressor at the transmitting station. This 1s particularly true 111 four wire systems 1n Vwhich it is vdesirable to maintain the same round trip singing equivalent at both high and low levels. ln systems in which voicel switched terminal circuits are usedv to get substantially one way operation it may be7 preferably not to provide an expander at the receiving end of the line. y

ln the accompanying drawings, Fig. 1 is a .diagrammatic view illustrating theoperation of volume control and volume compleSSOIl llleLnS: Figs. 2 and 3 are diagrammatic views indicating the positions of the volume control means and the volume compression means in different systems..

Fig. 4 is a diagrammatic view ofra` system me ans means included therein, a variable level source VLS comprising conductors 1 and 2 1s connected to an amplilier 3 by meansy of a potentiometer 4. The output ciroull Of the amplifier comprisingconductors 5, and 6 has the output level thereof n'iaintained constant and at a maximum level. Volume control means CM which is connected,to.the output conductors 5 and 6 governsthe operation of the potentiometer 4 the output from the amplifier stant maximumlevel. If the level -inthe output circuit tends to rise, theA control means 'CM operates the potentiometer 4 to govern the amplilier 3 and lower tlielevel in S at aA con- Vthe output circuit. If the level in the out.

autcircuit tends to fall below theiixed limit,rtl1e controlr means CM? operates the potentiometer 4 tov so control the amplifier 3 as to bring the output level upto the fixed maximum level. As an example of a system employing volume control means, attention is called to the patent to C. H. Fetter, 1,565,-

"f' 555 dated'Decembc-r 15, 1925.

In Fig.. 5 of the drawings is disclosed a system employing means for compressing the volumev range of the transmittery signals. A variable level source com-prisingl conductors 7 and 8 is connected to an amplifier 9. For the best operation of volume compression system of this type it is probably desirable to insert a delay circuit DC before the` amplifier 9. The amplifier 9 is connectedl tothe line conductors 10 and 11 to maintain Vwhich is received by the amplifier 9V fromy the.supplyrcondnctorsf and 8 is'compressed so that the volume range of the signals sup- 80 picd. .to the conductors-101cm; lflislliccdiiccd Referring to Fig.; 6.0i thcwdiioirins. transmission System iS,- indficatcd ,i which. 15: provided not only with volume,Y contr moons and` volumczccmpeossicn. transmitting stotioi.1,= beta1 evi/.ile volume/.f1- expanding meansY at ga' reeei ingylstation, transmitting Station 18 is; provided: with... volume control; circuits 119i oiiffchcdiy-Pezliid cated in Fig. 401ithed iiawingaioiiclwoliiicw9" compression circuitsr20lof thc ypc'indcotf ed, in Fig.A 5 ,of,the%dra.wings; .iimilalnpaltp in. Fig-1 6 tothoso .-showirdn Figsfl :ands 5iliavebcon indicated by like reference char A actors. At tho-.tr Smittngctati n; will bc @noted thc y@volume s controlj f circuits prcccdcfthc volume .compression :circuits 2 which. is preferable; ,for -reasons'fto-.r be .here inoftcr Set. fonth. lt is to, be. understood, howcvcathat if Soiflcsiricivohimo.compress 190 Sion. lineens. could 'bc zprovidedaataa lKarene. 1iiitt;i=1ig1.= stat-.ion before the volume; :cont-rolf' means... 1

At thoprccoiving stationl 21lshowii inFligc 6 of; the drawings, volume oxpandingfmeans. 195 22 similar to theexpandingameans shovs7 in. thc Methos-Patent No.v 11,151,729? are pilovicled, Theline conductorsio and; 11; at the receiving.; station; 21 ,are-Q-oonnected gto. hybridY coil@A '23 havng a.. Whoa/,tetona 1,10 bridge circuit; 24: formed .thereina One-amm 25. of the bridgecircuit hasfthe@iinpedancen thereof controlled. by, volume4- expanding; mcciisQG'-, Thevoluino cxpandingfnieaii'sf; is controlled-rom the tnansmttingastation. 1.15 18 to reverse theefunction'performed by; ,the volume compression.meansattli Y, m-tting'station. Thehybridzcoil 23 isfcoi nected to aV receivingcirciit:comprising@ doctors 27 m1111281l A Avchiiiio.compremio system of, the.v type;v shown in, Fig.A y6 is; .come pletely Idisclosed,` in fr the above .mentio-ned; patciltto R- G, MathcS,.-No.f1,157,729; ier-,vol ume compressionfsystemfis also; disolosedfin;

the patenty to H1 S,' .Hami1to.n, l5-565,441, 125

dated December 1c25.i e

A better-understanding ,of the. fiinctiofnalg characteristics of;1 the twov sry-stems shoiwulin Figs. 4: and: 5 ofi the drawings; beschr..

tained by, referenoefto the diagrammaii@ 1,39

view shown in Fig. 1 of the drawings. In the diagram shown in Fig. `1 the abscissa represents the variable level input to the systems shown in Figs. 4 and 5; and the ordinates of the diagram represent the output from the two systems shown in Figs. 4 and 5. The ordinates and abscissae are in terms of volume range on any suitable arbitrary scale. For example, the arbitrary scale may be in terms of transmission units corresponding to the ratio of the power supply to the power received in a system or the transmission unit equals 10 log lOl-l- The volume range of the input OP in Fig. 1 depends on a number of factors. The useful modulation range received from different types of toll circuits will ordinarily never be less than about 2O transmission units. The lengths OM and NP shown in the diagram are considered to vary from 30 to 40 transmission units and each represents the modulation range of speech from a toll circuit. The location of each modulation range of power level may be at a high average level or a low average level, depending upon whether the talker is a person who talks loudly or softly and also upon whether he is talking over a short or long subloop. The difference in level due to whether or not the talker speaks loudly or softly may be from 25 to 30 transmission units. The difference in level due to the electrical length of the toll circuit may be about 10 to 15 transmission units.

In many circuits as before mentioned, there is a definite range of power levels between which speech may be transmitted. There is an upper limit to this range caused either by the load-carrying capacity of the transmitter or some relay device in the circuit and a lower limit due to thepresence of extraneous interference'in the system. In some cases, it maybe desirable to pass over circuits on which are very severe restrictions,-signals which have overall differences in power level in the order of from to 70 transmission units. In the diagram shown in Fig. 1, the control range OM may indicate the range of a weak talker on a long loop circuit, while the distance NP may indicate the volume range of a loud talker on a short loop. If the input level range of OP were applied to a volume control system, the output levelwould be indicated by the straight line at 45o marked OC to produce an output OR. Y

Incase volume control means as shown in Fig. 4 of the drawings is used for controlling an input OP as shown in Fig. 1 of the drawings, the volume range OM of the weak talker over a long loop circuit would be raised to the level of the strong talker over a short loop circuit. The line OM may be assumed to belmoved towards the right to coincide with the line NP.` The weak and loud talkers have the same maximum level which is indicated on the 45 lineby the receiving station, is equal to the transmission units indicated by the distance OR.

In case the variable input indicated by the line OP is applied to a system including volume compression means, as shown in Fig. 5 of the drawings, the linear system considered with respect to the 45 line OO in Fig. 1 of the drawings will be replaced by some other characteristic, for example, indicated by the dotted linev OUF. For practical considerations, it is desirable to make the new characteristic a straight line as, for example, the line ODEF. In considering the application of a volume compression system, the volume range of a weak talker is assumed to be indicated by the line OM as above set forth, the volume range for a loud talker being indicated by the distance NP. The volume range of a weak talker after passing through the volume compression circuits will be reduced to the range OW and the volume range of the loud talker will be reduced to the range TS as indicated on the diagrams shown in Fig. 1. The volume ranges OW and TS in the output are obtained b applying volume ranges OM and NP to t e characteristic line ODEF. Y Assuming that both the volume control and the volume compression circuits operate in the most ideal fashion, a choice of one system as compared with the other systeml will depend on whether the overall compression range is greater or less than the useful modulation speech level of a single talker. The volume range to be transmitted when volume compression circuits are employed is indicated by the distance OS in Fig. 1 of the drawings and the volume range to be transmitted when volume control circuits are employed is indicated by the distance OR.

Whether the distance OS is greater or less il.

than the distance OR will indicate whether or not volume compression circuits or volume control circuits should be employed.

In many cases where the restrictions of the transmission system are very severe, it Iii may be impossible to obtain the most ideal transmission by using either volume control or volume compression means. In such cases in accordance with the inventionit is proposed to use both volume lcontrol and method which consists in moving the average volume levels of the received signals to maintain a maximum peak level in the received signals, in changing the transmitted signals to a reduced and the same volume range, and in expanding the signals at the receiving station.

9. In a transmission system of limited volume range having a transmitting station supplied With signals having diffe-rent average volume levels, the method which consists in first controlling and then compressing the volume range.

In Witness whereof, I hereunto subscribe my name this 27 th day of March, 1929.

ROBERT C. MATHES. 

